Mask or not they still have to get likeness rights. If an actor is credited as starring as a character legally(more or less) likeness rights attach to that characters depiction, mask or not. There was a lawsuit about this in the 80s where a studio tried to get around likeness rights by using shots of a character that didn’t show the face and a few other shady things. The union sued the studio and it’s been established law since then that if an actor physically portrays a character(and recently it’s mostly accepted of its a digital of the character as welll like Golem) then likeness rights attach.
And there’s no Deadpool movie rights with out Reynolds signing off.
They could have gotten deals with the other X-men actors like they did with Jackman had the actors agreed to it. We have been told by diamond select that Jennifer Lawrence flat said no to any likeness deals. It has been reported that Fassbender and Mcavoy said the same. Jackman is reported to have waived likeness rights for most his fox marvel films to help promote the film and characters. Howard Chan has publicly stated that they need to acquire individual likeness rights for the X-men films and used Mcavoy Prof X from First class as an example.